It is common practice in drilling an oil well to first install a steel pipe or casing in the borehole. This casing serves to seal off fluids from the borehole and to prevent sloughing off or caving in of the walls of the hole. The outermost casing, sometimes called a conductor pipe, is first installed and it is driven or cemented into place in the hole. It serves as a foundation or an anchor for all the subsequent drilling operations. The casinghead is the top end of the casing which protrudes above the surface. It is to the casinghead that control valves and flow pipes are attached. The conductor pipe can also be used to conduct drilling mud through loose layers of earth, such as sand.
In some applications, the conductor pipe also serves to prevent contamination of any fresh water strata which have been penetrated. When driven, conductor pipe is self-supporting, but it does not generally support its weight from the top, when it is cemented in.
When driven, the conductor pipe is installed by being driven into the ground by a pile driver. The impact of the pile driver can upset the upper end of the casing, and it usually does, causing it to be uneven and of varying thickness. A casing which is driven into the ground in this manner must be sufficiently stiff to resist the repeated compressive stress shocking caused by the pile driver. In order to provide the necessary stiffness and to resist the unevenness at the exposed end, a thicker pipe, called drive pipe, is used.
Drive pipe can have an outside diameter ranging up to 48 inches, with 26 to 30 inches being most common. Wall thickness varies from 1/2 to 2 inches, and roundness can vary from minus 1/2 to plus one percent of diameter. Usually, a flow control device, fittings and flow pipes are mounted on top of the drive pipe, in conjunction with a diverter system. This diverter system consists of various fittings and air actuated valves used to vent a low pressure gas kick in shallow wells.
Regardless of the type of equipment mounted atop a drive pipe, it is mounted to a flange on a casing head adaptor fitting that is currently welded to the drive pipe. Welding is used because the exposed end of the drive pipe casinghead is rough and uneven after being driven into the ground. Considerable machining would be required to clean up the casinghead and thread it, if a threaded fitting were used. Welding is much quicker and cheaper, so it is the method always used, at least on drive pipe casingheads.
When the casinghead adaptor is welded on the drive pipe, some disadvantages result. A joint must be preheated prior to welding and stress relieved upon completion of the weldment. Both of these operations are time consuming, using two to four hours of expensive rig time. They are also difficult to monitor and control, especially with respect to the rate of temperature rise and fall. Then, the adaptor must either be left in place and abandoned if the well is a dry hole, or it must be cut off the drive pipe and refurbished before being used again. This is time consuming and expensive. This casinghead adaptor is a heavy flanged fitting which not only supports equipment installed above it but also can support production casing suspended inside.
It would be advantageous to have an adaptor which could be mounted on a drive pipe for attachment of a diverter without the need for welding. The aforementioned technical problems associated with welding would be eliminated, and the cost of installation would be materially reduced. This would be particularly useful if the adaptor could be easily removed from a drive pipe and moved to a different well without being refurbished to any great extent. The capability of remaining on the well as the permanent casinghead is also desirable.